Geoscience Reference
In-Depth Information
exchange of matter, allows the entirety of the Earth system to be
evolutionarily flexible.
Thus, we acknowledge that the description of an ecosystem,
whatever its perimeter, does not just come down to its life forms. It
cannot limit itself to describing instantaneous states, but should also
describe the dynamics and retroactions, where living and non-living
entities enter simultaneously into play. We will, therefore, conclude
here that the term “biosphere” is equivalent to that of the Earth system
(or global planetary ecosystem) from the advent of early life on Earth
(around 4 billion years ago) to today, as such, for the greater part of its
history. But in order to avoid any confusion, we will restrict ourselves
to using a single expression: “Earth system”.
1.2. The ocean, from its origins
1.2.1.
Was there an ocean 4.4 billion years ago?
Isotopic studies carried out on zircons from Australia dating from
4.4 billion years ago [WIL 01] have given rise to the emergence of the
hypothesis that liquid water could have been present on the primitive
Earth from the end of its principal accretion phase, characterized by the
presence of a magma ocean resulting from the transformation of
gravitational energy into heat. The liquid water would have
accumulated on a solid crust that had only just cooled.
Even if this hypothesis still remains widely disputed, it is generally
acknowledged that the oceans were probably formed more than 4
billion years ago. Evidently, this early liquid water was in equilibrium
with an atmosphere much denser than that of today, of which the
pressure at ground level was several tens of times greater than that of
today. The original oceans therefore probably experienced a very hot
phase, in the order of several hundreds of degrees Celsius.
1.2.2.
The origin of water on Earth (4.5 - 4 billion years ago)
It is self-evident to say that water on Earth has an extraterrestrial
origin because this is true of all the elements that constitute our planet.
